MECHANISMS WHICH CONTROL VO2 NEAR VO2MAX - AN OVERVIEW

The symposium, ''Mechanisms Which Control VO2 Near VO2max'' led to a g eneral agreement that there is a variable impediment to the movement o f O-2 from the interior of the red cell to the interior of the mitocho ndrion. By changing the variables associated with O-2 delivery or by a ltering the conditions for muscle contractions, the effective O-2 diff using capacity for muscle can be altered as well. Because it is often measured as the ratio of VO2/PvO(2), it was suggested that this be ref erred to as O-2 conductance rather than diffusing capacity. In contras t to the wide range of O-2 conductance values found by the symposium c ontributors, a very narrow range of O-2 extraction values was found wh en VO2 was graphed against O-2 delivery. The only experimental values that departed from this relationship to any degree were those where he moglobin function was altered or if blood now was forced to extraordin ary high levels by a pump. The limits for VO2 in contracting isolated muscle are set not only by O-2 supply but by O-2 demand associated wit h stimulus patterns. Other intriguing and perhaps useful questions are : 1) What is the relative contribution of such factors as diffusional shunting, flow heterogeneity, red cell transit time, etc., to the appa rent O-2 conductance? 2) How is blood flow to contracting muscle contr olled? 3) How is contractile force adjusted to energy supply?